Improving Propeller Performance Using a Hybrid Airfoil Propeller Design

The primary design tool used for propellers is the Blade Element Momentum Theory or BEMT. Aerodynamic forces are calculated based on the local inflow conditions and the forces are then summed along the span of the blade to find the total forces and moments exerted on the propeller. BEMT then uses an iterative procedure to calculate the lift and drag leading to the optimum thrust and torque for the minimum induced losses. Previous investigations have shown that reducing the thrust production at the tip in a double break propeller design configuration leads to minimum induced drag and improved performance making the propeller more efficient and quieter. A series of tests were performed on a 21.5 in diameter KDE CF215 stock propeller and three custom designed propellers. All the custom propellers had a 21.5 inch diameter, a 2.25 inch chord which tapered to an oval tip. The design point was 5 lbf of thrust at 7,000 ft under static conditions. A baseline propeller was designed using an SG6043 airfoil and the standard BEMT method. A 50_94 double break propeller with the SG6043 airfoil was also designed as well as a hybrid 50_94 double break propeller with the SG6043 airfoil near the hub that transitions to an SG6041 airfoil near the tip. The reduced drag with the SG6041 leads to lower power required than the 50_94 with the SG6043 airfoil over the entire span. The hybrid configuration shows the most promise for both efficiency and a reduction in sound pressure level.